The composite color video signals that are conventionally broadcast, for example in the NTSC (National Television System Committee) format, contain not only picture information (luminance and chrominance components) but also timing information (vertical sync pulses and horizontal sync pulses) and other reference information (e.g. equalizing pulses and color burst). The horizontal sync pulse and the color burst both occur in the horizontal blanking interval, i.e., the interval between the active line times of consecutive horizontal scan lines. The horizontal sync pulse is a negativegoing pulse, the 50 percent point of the leading edge of the sync pulse being regarded as the horizontal sync point. The color burst follows the horizontal sync pulse in the horizontal blanking interval and comprises a sinusoidal wave having a frequency of 3.58 MHz. Immediately before and after the burst the signal is at blanking level. In accordance with EIA (Electronic Industries Association) standard RS-170A, the start of burst is defined by the zero-crossing (positive or negative slope) that precedes the first half-cycle of subcarrier that is 50 percent or greater of the burst amplitude. The color burst is used in the television receiver to control a phase-locked oscillator which generates a continuous wave at subcarrier frequency and is used to extract the chrominance information from the composite video signal.
The color burst is inserted into the blanking interval at the point of origin of the composite video signal, e.g. a video camera or a video tape recorder (VTR). The sync pulse and the color burst may be generated internally of the signal source, i.e. by a sync generator and an oscillator within the signal source, but it is more common, at least in the professional video field, for the studio to have a master sync and burst generator that distributes sync and burst to all the signal sources in the studio. This insures that all signal sources have the same sync and burst frequencies.
In the production of a television program, signals from various sources are combined in a production switcher under the supervision of the program director, and the resulting signal is either distributed, e.g. by cable or broadcast transmission, in real time or is recorded for subsequent distribution. In order to avoid degradation of the picture represented by the signal when there is a switch from one source to another, it is essential not only that the horizontal and vertical sync pulses of the different signals arriving at the switcher should be in phase but also that the signals be color framed, i.e. that the fields of the signals be in phase. This implies that the color bursts of the different signals should be in phase. Accurate phasing of the color bursts cannot normally be achieved simply by use of a master burst generator, because different cable lengths between the signal sources and the switcher result in different signal transmission times, so that even if the color bursts of all the signals are in phase at the signal sources, it is almost certain that the color bursts of the different signals will not all be in phase at the switcher.
The conventional method for bringing the color bursts of the different program video signals into phase at the switcher involves the use of a vectorscope and a "black burst" reference signal. A first program video signal is applied to the video A input of the vectorscope and the reference signal is applied to the video B input. The vectorscope provides a display of a vector at an angle that depends upon the phase difference between the color burst of the first program signal and the black burst. The goniometer of the vectorscope is adjusted to align this vector with a graticule mark. The first program video signal is then disconnected and a second program video signal is applied to the video A input, and the vectorscope displays a vector at an angle that is representative of the phase difference between the burst of the second program video signal and the black burst. The phase of the burst of the second program video signal is adjusted at the signal source to align the vector with the graticule mark that the vector generated in the first comparison was aligned with. The burst of the second program video signal is then in phase with the burst of the first program video signal. This operation, which is repeated for each of the program video signals, is known as A/A phasing, to distinguish it from A/B phasing, in which two program video signals are applied to the video A input and the video B input respectively of the vector-scope. A/A phasing is normally preferred over A/B phasing, which requires matching of the cable lengths to the vectorscope, because with A/A phasing the same cable is used for each comparison and therefore the path delay or phase shift to the vectorscope does not affect the comparison.
The method described above for bringing the bursts of the different program video signals into phase at the switcher is subjective, in that it depends upon the view of the operator whether the vector generated in the second or subsequent comparison is correctly aligned with the appropriate graticule mark. Moreover, the operator must adjust the goniometer carefully on the first signal and make sure that its setting has not been changed when the second signal is tested.